Continued investigation at the ultrastructural level on the extent of resin penetration into treated enamel surfaces and the effectiveness of the seal against further acid attack. Examination of specific areas of enamel which show structural variations. A correlated scanning and transmission microscopic approach will be used. The effect of removal of the organic matrix of enamel will be investigated. A correlated microradiographic and transmission electron microscopic study of sections cut through a carious lesion will be initiated. Use of selected area diffraction will be continued to determine if there are any variations in the enamel crystal population.